1. Field of the Invention
This invention relates to a machining method using hydroxide ions in ultrapure water. More particularly, the invention relates to an electrolytic machining method and an electrolytic machining apparatus which use only ultrapure water as an electrolytic solution and increase its ion product markedly, thus being capable of removal processing a workpiece or forming an oxide film thereon by the action of hydroxide ions.
2. Description of the Related Art
In recent years, new materials have been developed one after another with the progress of science and technology. However, effective processing techniques for these new materials have not been established, and constant pursuit after forerunning new materials under development has been encouraged. Recently, microstructure and high precision have been introduced into components of every instrument. As fabrication on the submicron scale has become common, processing methods themselves have exerted increasing influence on the characteristics of materials. Under these circumstances, a processing method in which a tool effects removal processing of a workpiece while physically destroying it, such as conventional machining, causes many defects to the workpiece. As a result, the characteristics of the workpiece are deteriorated. How to process a material without impairing its characteristics poses a problem.
Among special processing methods initially developed as means for solving the problem are chemical polishing, electrolytic machining, and electrolytic polishing. These processing methods, as contrasted with conventional physical processing, carry out removal processing by causing a chemical dissolution reaction. Hence, defects due to plastic deformation, such as affected layers or dislocations, do not occur, and the above-mentioned challenge of processing a material without impairing its characteristics can be solved.
More attention has been paid to a processing method which makes use of a chemical interaction between atoms. This is a processing method utilizing fine particles or a radical with high chemical reactivity. Such a processing method performs removal processing through a chemical reaction, on the atomic order, with a workpiece. Thus, it is capable of processing control on the atomic order. Examples of this processing method are EEM (elastic emission machining) and plasma CVM (chemical vaporization machining) which were developed by the inventor of the present invention. EEM utilizes a chemical reaction between fine particles and workpiece, and realizes processing on the atomic order, without impairing the characteristics of the material. Plasma CVM utilizes a radical reaction between radicals generated in atmospheric plasma and workpiece, and realizes processing on the atomic order.
With the aforementioned electrolytic machining or electrolytic polishing, processing has hitherto been said to proceed by the electrochemical interaction between the workpiece and an electrolytic solution (an aqueous solution of NaCl, NaNO3, HF, HCl, HNO3 or NaOH). As long as the electrolytic solution is used, contamination of the workpiece with the electrolytic solution is unavoidable.
The inventor estimated that in a neutral or alkaline electrolytic solution, hydroxide ions (OHxe2x88x92) must take part in processing, and speculated that processing would be possible with water containing traces of hydroxide ions. Experimentally, the inventor confirmed the possibility for such processing. Based on the findings, the inventor proposed in Japanese Unexamined Patent Publication No. 58236/1998 a method which uses only ultrapure water except for traces of incidental impurities, and applies thereto a hydroxide ion increasing treatment for increasing its ion product. According to this method, a workpiece immersed in ultrapure water having an increased concentration of hydroxide ions is subjected to a chemical dissolution reaction or an oxidation reaction with hydroxide ions, whereby removal processing or oxide film formation takes place. As the hydroxide ion increasing treatment, the inventor also proposed the use of an electrochemical reaction occurring on a solid surface having an ion exchange function or a catalytic function. These proposals have led to the creation of a novel processing method capable of clean processing, with no impurities left behind on the processed surface, by utilizing hydroxide ions in ultrapure water. This processing method is expected to have wide varieties of applications, including the production of semiconductors.
However, it is a well known fact that the concentration of hydroxide ions in ultrapure water is very low and about 10xe2x88x927 mol/l at 25xc2x0 C. and 1 atmosphere. Even with the use of a conventional ion exchange membrane, the hydroxide ion density increased thereby is at most about 10 3- to 104-fold. This value is 1/104 to 1/103 of the ion density of 1N NaOH. It would make the processing speed still too low to realize practical processing.
In light of the foregoing circumstances, the present invention aims to provide a processing method capable of performing clean processing, with no impurities left behind on a processed surface of a workpiece, with the use of hydroxide ions in ultrapure water, which method further increases an oxide ion density on a processed surface of a workpiece, and also rapidly removes atoms of the workpiece, bound to the hydroxide ions, from the processed surface, thereby raising the processing speed.
A first aspect of the invention, for attaining the above-mentioned object, is an electrolytic machining method comprising placing an anode and a cathode in ultrapure water, the anode serving as a workpiece, and the cathode being opposed to the anode with a predetermined spacing; disposing a catalytic material between the workpiece and the cathode, the catalytic material promoting dissociation of the ultrapure water and having water permeability; and forming a flow of the ultrapure water inside the catalytic material while applying a voltage between the workpiece and the cathode, to decompose water molecules in the ultrapure water into hydrogen ions and hydroxide ions, and supply the resulting hydroxide ions to a surface of the workpiece, thereby performing removal processing of or oxide film formation on the workpiece through a chemical dissolution reaction or an oxidation reaction mediated by the hydroxide ions.
FIG. 1 shows the principle of processing according to the invention. In ultrapure water, a workpiece 2 as an anode, and a cathode 1 opposed thereto are placed. Between the workpiece 2 and the cathode 1, a nonwoven fabric 3 having ion exchange ability as a catalytic material is disposed. A power source 4 is connected to the workpiece 2 and the cathode 1, and water molecules, a, in the ultrapure water are decomposed into hydroxide ions, b, and hydrogen ions, c, by the ion exchange material 3. The resulting hydroxide ions, b, are supplied to a surface of the workpiece by an electric field working between the workpiece 2 and the cathode 1 and by a flow of the ultrapure water to increase the density of the hydroxide ions near the workpiece and react the hydroxide ions, b, with atoms, d, of the workpiece. Reaction products, e, formed by the reaction dissolve into the ultrapure water, and are removed from the workpiece 2 by the flow of the ultrapure water along the surface of the workpiece 2. In this manner, removal processing of a surface layer of the workpiece 2 is carried out. Alternatively, an oxidation reaction between the workpiece atoms d and the hydroxide ions b forms a clean oxide film on the surface of the workpiece, thereby performing processing for oxide film formation. Accumulation of this film can obtain the desired shape.
According to the above-described method, hydroxide ions b are formed by the chemical reaction on the solid surface having ion exchange function or catalytic function that is installed in the vicinity of the surface of the workpiece. Thus, the surface of the workpiece close to the surface of the solid material that generates such hydroxide ions b is preferentially processed. Movement of this part that proceeds with processing enables the surface of the workpiece 2 to be processed into a desirable surface shape.
So-called transfer processing is also possible in which the shape of the ion exchange material 3 that generates hydroxide ions b is transferred to the surface of the workpiece. If the shape of the ion exchange material 3 that generates hydroxide ions b is linear, a plate-like material can be cut. By adjusting processing parameters, such as the supply amount of hydroxide ions b, moreover, it becomes possible to select whether the reaction induced on the surface of the workpiece 2 will be an oxidation reaction, or a removal processing reaction.
The present invention using such a catalytic material, and a conventional method using an ion exchange membrane will be compared and explained. With the conventional method, as shown in FIG. 2, hydrogen ions, c, can move into an ion exchange membrane 3a, but hydroxide ions b and water molecules a minimally move into its interior. Thus, hydroxide ions b formed by electrolysis of water occur mainly on the surface of the ion exchange membrane 3a, and active points inside the ion exchange membrane remain unused. Hence, the efficiency of formation of hydroxide ions b in the ion exchange membrane 3a was not sufficient, and an electric current flowing between the two electrodes was low. On the other hand, the present invention uses a water permeable catalytic material having some volume, such as a nonwoven fabric 3b given ion exchange ability. According to this invention, as shown in FIG. 3, water flows sufficiently into the nonwoven fabric 3b, so that active points inside the nonwoven fabric 3b can be utilized for electrolysis of water. In addition, the hydroxide ions b generated inside the nonwoven fabric can easily arrive at the workpiece as the anode as water moves. In the invention, therefore, a high current density difficult to achieve by the conventional method can be obtained easily.
FIGS. 1 to 3 are concept views schematically showing a processing apparatus for realizing a processing method relying on hydroxide ions in ultrapure water according to the invention. This apparatus will be explained in further detail by reference to FIG. 4, but the invention is not restricted thereby.
In the electrolytic machining method as the first aspect of the invention, the catalytic material may be a nonwoven fabric provided with ion exchange ability. Such a nonwoven fabric is prepared by radiation-induced graft polymerization of a nonwoven fabric having a suitable fiber diameter and a void ratio, namely, a polymerization method which irradiates the nonwoven fabric with, for example, xcex3 rays and then graft polymerizes the irradiated fabric. As the catalytic material, a cloth of ion exchange fibers, or a net having ion exchange groups introduced therein can be named. The ion exchange groups of the nonwoven fabric provided with ion exchange ability are desirably strongly basic anion exchange groups, or strongly acidic cation exchange groups.
As regards the gap between the nonwoven fabric and the cathode, or between the nonwoven fabric and the workpiece (anode), it is advantageous that the nonwoven fabric be in contact with both electrodes, because the value of the current can be increased. However, the reaction product tends to build up between the electrode and the nonwoven fabric, and processing may become nonuniform. Increasing the flow velocity of ultrapure water can resolve this possibility, but is not preferred, because an increase in a pressure loss of the apparatus is not avoidable. To remove the reaction product rapidly from the workpiece and the cathode, therefore, it is preferred to provide a gap between the nonwoven fabric and the electrode, especially the workpiece. The manner of providing the gap, and the size of the gap are selected according to the purpose of processing.
In the above electrolytic machining method, the catalytic material may be disposed in contact with one of the cathode and the anode, and away from the other.
In the above electrolytic machining method, the catalytic material may be disposed between the workpiece (anode) and the cathode so as to contact both of the workpiece and the cathode.
In the above electrolytic machining method, electrolytic machining may be performed, with ultrapure water being flowed in one direction between the workpiece and the cathode. According to this feature, the velocity of a flow of ultrapure water between the cathode and the workpiece, including the nonwoven fabric, is adjusted, whereby metal ions formed by the reaction can be removed or controlled reliably according to the purpose of processing or the characteristics of the workpiece.
A second aspect of the invention is an electrolytic machining apparatus, comprising a processing tank for holding ultrapure water, means for bearing a cathode and a workpiece in the processing tank, means for fixing a nonwoven fabric provided with ion exchange ability between the workpiece and the cathode, means for applying a voltage between the cathode and the workpiece, an ultrapure water feeder for supplying ultrapure water into the processing tank, and means for discharging waste water, which contains impurities formed by electrolytic machining, to the outside of the processing tank.
In accordance with the foregoing features of the invention, a flow of ultrapure water is formed inside a nonwoven fabric provided with ion exchange ability, whereby hydroxide ions are formed in much larger an amount than by a conventional ion exchange material such as an ion exchange membrane. The hydroxide ions can be supplied efficiently to the surface of a workpiece by an electric field and the flow of ultrapure water. Thus, the density of hydroxide ions near the surface of the workpiece is increased. Also, reaction products formed by the reaction between hydroxide ions and atoms of the workpiece are rapidly removed from the surface of the workpiece by the flow of ultrapure water. As a result, a fresh surface to be processed is always exposed, so that the processing speed becomes dramatically fast.
Furthermore, processing takes place by the electrochemical action of hydroxide ions and workpiece atoms. Thus, the workpiece is prevented from suffering physical defects and impaired characteristics. Besides, processing in ultrapure water can prevent contamination with impurities from the outside world, and can lead to the provision of a processed article having high quality. In addition, only ultrapure water is used, so that a burden on waste water disposal is very light, and a marked decrease in the processing cost is possible.